llvm dev - Feb 2017 - llc -O3 generates basic blocks that end without jump

Hello.
     I noticed that when I pass the -O3 flag to llc some of the resulting ASM
basic blocks
(as seen by the AsmPrinter phase and the resulting .s file) don't contain
any jump, be it
conditional or unconditional. For example I get this ASM .s file after running
llc -O3:
       // BB#0:                                // %entry
         mov     r0, 0
         ld_64 r1, ...
         mov     r2, 256
         // Notice there is no jump
       LBB0_1:                                 // %for.cond2.preheader
         mov     r3, 0
         ...

     These are not really 2 basic blocks, so the first block without jump should
be merged
with the successor block (if just one successor, which is always the case as far
as I can
see on the tests I ran).

     Did anybody already address such correction of "malformed" basic
blocks obtained with
llc -O3 (or -O1/O2)? Or is there any other flag that I am missing when invoking
llc?

   Thank you,
     Alex

> On Feb 7, 2017, at 16:16, Alex Susu via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hello.
>    I noticed that when I pass the -O3 flag to llc some of the resulting ASM
basic blocks (as seen by the AsmPrinter phase and the resulting .s file)
don't contain any jump, be it conditional or unconditional. For example I
get this ASM .s file after running llc -O3:
>      // BB#0:                                // %entry
>        mov     r0, 0
>        ld_64 r1, ...
>        mov     r2, 256
>        // Notice there is no jump
>      LBB0_1:                                 // %for.cond2.preheader
>        mov     r3, 0
>        ...
> 
>    These are not really 2 basic blocks, so the first block without jump
should be merged with the successor block (if just one successor, which is
always the case as far as I can see on the tests I ran).
> 
>    Did anybody already address such correction of "malformed"
basic blocks obtained with llc -O3 (or -O1/O2)? Or is there any other flag that
I am missing when invoking llc?
> 
>  Thank you,
>    Alex
> 
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> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
BranchFolding will attempt to replace unconditional jumps with fallthroughs
where applicable. A branch instruction is not needed at the end of a
MachineBasicBlock. I’m not aware of anything that will attempt to merge these
blocks into one (and I don’t see much of a reason to do this, there shouldn’t be
much of difference in the final code unless maybe blocks are padded with nops
for aligning them). There are a few situations where the branch label will be
emitted even though there are no branches to that block. See where
AsmPrinter::isBlockOnlyReachableByFallthrough is used

-Matt

On 7 February 2017 at 16:16, Alex Susu via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>     These are not really 2 basic blocks, so the first block without jump
> should be merged with the successor block (if just one successor, which is
> always the case as far as I can see on the tests I ran).
Like Matt, I don't see much benefit in merging these or think they're
malformed (they're just a trivial configuration for part of the CFG).

In many cases this structure will actually be necessary since LLVM
blocks can only have one entry point. So if some other block jumps to
LBB0_1 (in your case), they have to remain separate.

Cheers.

Tim.